The present studies are concerned with the role of 5-HT in neural functioning in the leech. Work in progress is on the identification of neurons that inhibit body wall muscle in the leech. Work will be concerned with determining whether these neurons store large amounts of 5-HT or whether other compounds are present in these cells. Other work in progress will concern the function of known 5-HT containing cells in this nervous system. Bibliographic references: Coggeshall, R.E., M.L. Applebaum, M. Fazen, R.B. Stubbs, III, and M.T. Sykes. 1975. Unmyelinated ventral roots of humans, a possible explanation for the failure of the dorsal rhizotomy to relieve pain. Brain, 98: 157-166. Wilkinson, J.M. and R.E. Coggeshall. 1975. Axonal numbers and sizes in the connectives and peripheral nerves of the leech. J. Comp. Neurol., 163: 387-396.